Optical sensor

ABSTRACT

An optical sensor for monitoring an environmental condition, the optical sensor comprising a perfluorosulfonate ionomer membrane comprising a solution, wherein the solution comprises a transition metal-free dye component, wherein exposure of the optical sensor to a specific environmental condition produces a color shift on the optical sensor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from provisional patent applicationSer. No. 60/077,513, filed Jul. 2, 2008.

TECHNICAL FIELD

The present invention relates generally to optical sensors and methodsof manufacturing optical sensors.

BACKGROUND OF THE INVENTION

Diacetyl (2,3-butanedione; C₆H₆O₂) and trimellitic anhydride (“TMA”;1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxylic acid; C₉H₄O₅) are toxicagents. Exposure to either may result in the development of respiratoryillness or disease. Historically, optical detection and analysis oftoxic agents has been more practical than other means for on-linecontinuous monitoring of hazardous levels because of the rapidity of theanalysis. Optical detection and analysis of diacetyl has previouslyemployed transitional metal complexes precipitated onto filters forvisual inspection or applied to transparent films forultraviolet-visible (“UV/VIS”) spectroscopy. However, transitional metalcomplexes are expensive, difficult to prepare, and not sufficientlydurable for continuous monitoring applications in clinical ormanufacturing settings.

At present, there are no practical methods of rapid, on-line, continuousmonitoring of either diacetyl or TMA levels.

There is a need in the industry for optical sensors capable of detectingthe presence of diacetyl, TMA, and other elements or compounds, that aremore durable, less expensive, and easier to manufacture than thespectroscopic filters and films currently available.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides an optical sensor formonitoring an environmental condition.

One embodiment of the invention provides an optical sensor formonitoring an environmental condition, the optical sensor comprising aperfluorosulfonate ionomer (“PFSI”) membrane comprising a solution,wherein the solution comprises a transition metal-free dye component,wherein exposure of the optical sensor to a specific environmentalcondition produces a color shift on the optical sensor.

Another embodiment of the invention provides a method of monitoring anenvironmental condition with an optical sensor comprising aperfluorosulfonate ionomer membrane comprising a solution, wherein thesolution comprises a transition metal-free dye component, the methodcomprising the steps of exposing the optical sensor in an environmentand examining the optical sensor for color shift associated with aspecific environmental condition.

Another embodiment of the invention provides a method of manufacturingan optical sensor for monitoring an environmental condition, the methodcomprising the steps of: preparing a solution, wherein the solutioncomprises a transition metal-free dye component; immersing aperfluorosulfonate ionomer membrane in the solution; and removing themembrane after it has absorbed the solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the UV/VIS spectrum of anoptical sensor comprising a PFSI membrane and 3,4-diaminobenzophenoneafter exposure to a 30 ppm diacetyl vapor (after subtraction of theoptical sensor spectrum).

FIG. 2 is a black-and-white photograph of optical sensors comprisingPFSI membranes and aromatic-diamine before and after exposure to aglucose solution.

DETAILED DESCRIPTION OF THE INVENTION Definitions

“Color shift” is a change in the absorption or reflection ofelectromagnetic radiation. Color shift need not be detectable by thenaked eye.

Exposure of an aromatic-diamine dye to diacetyl in the presence of anacid catalyst results in an irreversible color shift of thearomatic-diamine in the visible region of the electromagnetic spectrum.That is, aromatic-diamine that has not been exposed to diacetyl in thepresence of an acid catalyst is different in color than aromatic-diaminethat has been exposed.

Perfluorosulfonate ionomer (“PFSI”) membranes provide media that caneffectively and irreversibly absorb aromatic-diamine from solution. ThePFSI membrane provides support for the aromatic-diamine dye and PFSIserves as the acid catalyst necessary to activate the aromatic-diamine.When exposed to diacetyl, the PFSI membrane comprising aromatic-diamineexhibits an irreversible color shift, thereby serving as an opticalsensor capable of effectively detecting the presence and relativeconcentration of diacetyl in an environment.

The optical sensor color is dependent upon the concentration ofaromatic-diamine in the optical sensor. The magnitude of the opticalsensor's color shift upon exposure to diacetyl is dependent upon theconcentration of aromatic-diamine in the optical sensor, as well as theconcentration of diacetyl in the environmental in which the opticalsensor is exposed. The sensitivity of the optical sensor can be tunedfor desired diacetyl concentrations and environments by adjusting thearomatic-diamine concentration of the optical sensor. The optical sensorhas exhibited sensitivity to diacetyl concentrations as low as parts perbillion.

Optical sensors comprising PFSI membranes and aromatic-diamine are alsocapable of detecting and discriminating, via color shift, amongaldehydes and ketones that are chemically similar to diacetyl, such asvinyl acetate and 2,3-pentanedione.

Optical sensors comprising PFSI membranes and aromatic-diamine alsoexhibit sensitivity to ionic platinum. Ionic platinum is anthropogenicand may pose a human health hazard. Metallic platinum serves as anelectrocatalyst in proton exchange membrane (“PEM”) fuel cell electrodesand is widely employed in automobile catalytic converters. Thedegradation and dissolution of metallic platinum during typicaloperation yields ionic platinum. Optical sensors comprising PFSImembranes and imbibed aromatic-diamine may be used to monitor thedegradation process. Information obtained during such monitoring may beused to improve electrode durability and mitigate beginning of lifeefficiency losses resulting from mass transport limitations.

Optical sensors comprising PFSI membranes and aromatic-diamine alsoexhibit sensitivity to glucose. An unexposed optical sensor undergoes acolor shift upon exposure to glucose. Detection of glucose with theoptical sensor of this invention may be useful in human healthmonitoring and on-line monitoring of sugar content during biofuelprocessing.

PFSI membranes comprising aromatic-diamine can also effectively sense pHlevels. PFSI membranes can effectively absorb azo-based dyes fromsolutions containing azo-based dyes. When such membranes absorbazo-based dye, they become optical sensors capable of sensing pH levelsvia color shift. Optical sensors capable of sensing pH levels areimportant for monitoring breath condensate, an important mediator inasthma.

Exposure of an aromatic dye to an acid anhydride in the presence of anacid catalyst results in alteration of the electromagnetic absorptionspectrum of the dye. An aromatic dye that has not been exposed to ananhydride in the presence of an acid catalyst is different in color thanan aromatic that has been exposed to an anhydride in the presence of anacid catalyst.

PFSI membranes provide media that can effectively absorb an aromaticdye, such as benzene-1,3-diol (also known as resorcinol), from solution.The PFSI membrane provides support for the benzene-1,3-diol and alsoserves as the acid catalyst necessary to alter the light(electromagnetic) absorption spectrum of the dye in the presence of ananhydride, such as TMA. When exposed to TMA, the PFSI membranecomprising the benzene-1,3-diol exhibits an irreversible color shift,thereby serving as an optical sensor capable of effectively detectingthe presence and relative concentration of TMA in an environment.

Exposure of the optical sensor to solid or liquid TMA yields an opticalresponse comparable to that exhibited by the optical sensor when exposedto the vapor phase of TMA.

Optical sensors comprising PFSI membranes and benzene-1,3-diol may alsobe used to effectively detect the presence and relative concentration ofphthalic anhydride or maleic anhydride, via color shift.

The magnitude of the optical sensor's color shift upon exposure of theoptical sensor to TMA, phthalic anhydryide, or maleic anhydride isdependent upon the concentration of benzene-1,3-diol in the opticalsensor, as well as the concentration of TMA, phthalic anhydride, ormaleic anhydride in the environment to which the optical sensor isexposed. The sensitivity of the optical sensor can be tuned to detectlower or higher concentrations of anhydrides by adjusting thebenzene-1,3-diol concentration of the optical sensor. The optical sensorhas exhibited sensitivity to TMA, phthalic anhydride, or maleicanhydride concentrations as low as parts per billion.

The color shift associated with the presence of diacetyl, vinyl acetate,2,3-pentanedione, TMA, phthalic anhydride, maleic anhydride, ionicplatinum, or glucose or pH level may be detected with the naked eye.More detailed analysis of exposure over time is possible usingspectroscopic analysis, which can analyze the color intensity variationover time. Such spectroscopic analysis can be performed on a real-timecontinuous basis using a portable monitor with light-emitting diodelight sources and charge-coupled devices for analysis of the intensityof adsorbed and transmitted light. Alternatively, optical sensors may becollected periodically and analyzed at off-site spectrophotometers.

Exemplary embodiments of an optical sensor for monitoring anenvironmental condition are hereinafter described in detail inconnection with the views and examples of FIGS. 1-2.

One exemplary embodiment of the invention comprises an optical sensorfor monitoring an environmental condition comprising aperfluorosulfonate ionomer membrane comprising a solution, wherein thesolution comprises a transition metal-free dye component, whereinexposure of the optical sensor to a specific environmental conditionproduces a color shift on the optical sensor.

In a specific exemplary embodiment of the optical sensor, the solutioncomprises aromatic-diamine. In a more specific exemplary embodiment ofthe optical sensor the aromatic-diamine comprises3,4-diaminobenzophenone.

In another specific exemplary embodiment of the optical sensor, thesolution comprises an azo-based dye.

In another specific exemplary embodiment of the optical sensor, thesolution comprises benzene-1,3-diol.

In another specific exemplary embodiment of the optical sensor, themonitored specific environmental condition is the presence of diacetyl.

In another specific exemplary embodiment of the optical sensor, themonitored specific environmental condition is the presence of vinylacetate.

In another specific exemplary embodiment of the optical sensor, themonitored specific environmental condition is the presence of2,3-pentanedione.

In another specific exemplary embodiment of the optical sensor, themonitored specific environmental condition is the presence oftrimellitic anhydride.

In another specific exemplary embodiment of the optical sensor, themonitored specific environmental condition is the presence of phthalicanhydride.

In another specific exemplary embodiment of the optical sensor, themonitored specific environmental condition is the presence of maleicanhydride.

In another specific exemplary embodiment of the optical sensor, themonitored specific environmental condition is the presence of ionicplatinum.

In another specific exemplary embodiment of the optical sensor, themonitored specific environmental condition is the presence of glucose.

In another specific exemplary embodiment of the optical sensor, themonitored specific environmental condition is pH level in breathcondensate.

Another exemplary embodiment of the present invention comprises a methodof monitoring an environmental condition with an optical sensorcomprising a perfluorosulfonate ionomer membrane comprising a solution,wherein the solution comprises a transition metal-free dye component,the method comprising the steps of exposing the optical sensor in anenvironment and examining the optical sensor for color shift associatedwith a specific environmental condition.

In a specific exemplary embodiment of the method of monitoring anenvironmental condition with an optical sensor, examining comprisesvisually inspecting the optical sensor.

In another specific exemplary embodiment of the method of monitoring anenvironmental condition with an optical sensor, examining comprisesspectroscopic analysis.

In another specific exemplary embodiment of the method of monitoring anenvironmental condition with an optical sensor, monitoring occurscontinuously at ambient conditions.

Another specific exemplary embodiment of the method of monitoring anenvironmental condition with an optical sensor further comprisesemitting an alarm when the optical sensor indicates a specificenvironmental condition.

Another exemplary embodiment of the present invention comprises a methodof manufacturing an optical sensor for monitoring an environmentalcondition, the method comprising the steps of preparing a solution,wherein the solution comprises a transition metal-free dye component;immersing a perfluorosulfonate ionomer membrane in the solution; andremoving the membrane after it has absorbed the solution.

In a specific exemplary embodiment of the method of manufacturing anoptical sensor, preparing the solution comprises dissolving anaromatic-diamine in alcohol. In a more specific embodiment, thearomatic-diamine comprises 3,4-diaminobenzophenone.

In another specific exemplary embodiment of the method of manufacturingan optical sensor, preparing the solution comprises dissolving anazo-based dye in alcohol.

In another specific exemplary embodiment of the method of manufacturingan optical sensor, preparing the solution comprises dissolvingbenzene-1,3-diol in alcohol.

EXAMPLE 1

In this example, a solution comprising a transition metal-free dyecomponent is prepared by dissolving 120 mg of 3,4-diaminobenzophenone in12.5 ml of ethanol. A PFSI membrane (e.g., Nafion® 1100EW, availablefrom DuPont™) is immersed in the solution for at least 24 hours, untilno further color change is observed. Before immersion, the membrane istransparent. When removed from the solution, the membrane is bright red.

A diacetyl solution is prepared by combining 5 microliters of diacetyland 0.5 ml of water together. Approximately 5 microliters of thediacetyl solution is then transferred with a pipette into a 500 ml roundbottom flask.

The dyed, bright red PFSI membrane, now an optical sensor, is tied withparafilm to one end of a gold-coated stainless steel strip and suspendedin the flask above the diacetyl solution contained within the flask. Theflask is immediately sealed with a rubber stopper, which pins the steelstrip against the mouth of the flask and leaves the optical sensorsuspended above the diacetyl solution. The stopper is sealed withparafilm.

The bottom of the flask is heated with a hot plate to vaporize thediacetyl solution and expose the optical sensor to 30 ppm diacetylvapor. The flask is removed from the hot plate after 15 minutes. Afterexposure to the diacetyl vapor, the PFSI membrane is dark red. The colorshift from bright red, before diacetyl exposure, to dark red, afterdiacetyl exposure, is irreversible.

FIG. 1 illustrates the UV/VIS spectrum of the optical sensor followingexposure to the diacetyl vapor, after subtraction of the optical sensorspectrum. A prominent peak occurs at about 590 nm, a substantial shiftfrom the pre-exposure peak of about 350 nm.

EXAMPLE 2

In this example, a solution comprising a transition metal-free dyecomponent is prepared by dissolving 8 mg of 3,4-diaminobenzophenone in12.5 ml of ethanol. A PFSI membrane is immersed in the solution for atleast 24 hours, until no further color change is observed. When removedfrom the solution, the membrane is yellow-green.

The dyed, yellow-green PFSI membrane, now an optical sensor, is exposedto 30 ppm diacetyl vapor using the procedure set forth in Example 1.After exposure to the diacetyl vapor, the PFSI membrane is dark green.The color shift from yellow-green, before diacetyl exposure, to darkgreen, after diacetyl exposure, is irreversible.

EXAMPLE 3

In this example, a solution comprising a transition metal-free dyecomponent is prepared by dissolving 24 mg of 3,4-diaminobenzophenone in12.5 ml of ethanol. A PFSI membrane is immersed in the solution for atleast 24 hours, until no further color change is observed. When removedfrom the solution, the membrane is light orange.

The dyed, light orange PFSI membrane, now an optical sensor, is exposedto 30 ppm diacetyl vapor using the procedure set forth in Example 1.After exposure to the diacetyl vapor, the PFSI membrane is dark orange.The color shift from light orange, before diacetyl exposure, to darkorange, after diacetyl exposure, is irreversible.

EXAMPLE 4

In this example, a solution comprising a transition metal-free dyecomponent is prepared by dissolving 60 mg of 3,4-diaminobenzophenone in12.5 ml of ethanol. A PFSI membrane is immersed in the solution for atleast 24 hours, until no further color change is observed. When removedfrom the solution, the membrane is light red.

The dyed, light red PFSI membrane, now an optical sensor, is exposed to30 ppm diacetyl vapor using the procedure set forth in Example 1. Afterexposure to the diacetyl vapor, the PFSI membrane is dark red. The colorshift from light red, before diacetyl exposure, to dark red, afterdiacetyl exposure, is irreversible.

EXAMPLE 5

In this example, a solution comprising a transition metal-free dyecomponent is prepared by dissolving 8 mg of 3,4-diaminobenzophenone in12.5 ml of ethanol. A PFSI membrane is immersed in the solution for atleast 24 hours, until no further color change is observed. When removedfrom the solution, the membrane is light green.

Different portions of the dyed, light green PFSI membrane, now anoptical sensor, are exposed to various concentrations of diacetyl vapor(5, 10, 15 and 20 ppm) using the procedure set forth in Example 1, withthe exception that portions of the optical sensor exposed to oneconcentration of diacetyl vapor are not exposed to other concentrations.Exposure of the optical sensor to the various concentrations of diacetylvapor results in the color of the exposed portions shifting from lightgreen to darker green, with increased concentrations of diacetyl vaporresulting in darker shades of green. A color shift is observed even onthe portion of the optical sensor exposed to a diacetyl vaporconcentration of 5 ppm.

EXAMPLE 6

In this example, two optical sensors are prepared according to theprocedure described in a Example 1. Both optical sensors are bright red.One optical sensor is exposed to a 10 ppm concentration of vapor of thediacetyl isomer, vinyl acetate and the other optical sensor is exposedto a 10 ppm concentration of vapor of the diacetyl-like diketone,2,3-pentanedione, using the procedure set forth in Example 1. Theoptical sensor exposed to vinyl acetate exhibits a color shift frombright red to reddish-brown; the optical sensor exposed to2,3-pentanedione exhibits a color shift to a darker shade of red thanthat of the pre-exposure optical sensor.

EXAMPLE 7

In this example, multiple solutions comprising transition metal-free dyecomponents are prepared by dissolving 120 mg of 3,4-diaminobenzophenonein 12.5 ml of 50/50 ethanol/water solutions (by volume) at various pHlevels. A separate PFSI membrane is immersed into each solution for aperiod of 24 hours. The PFSI membranes are optically sensitive to pHlevel. For example, the PFSI membrane immersed in the solution with a pHlevel of 3 exhibited a color shift to bright red; the PFSI membraneimmersed in the solution with a pH level of 6 exhibited a color shift tobrown-yellow; the PFSI membrane immersed in the solution with a pH levelof 8.5 exhibited a color shift to olive green.

EXAMPLE 8

In this example, a solution comprising a transition metal-free dyecomponent is prepared by dissolving 0.12 grams of benzene-1,3-diol (alsoknown as resorcinol) in 12.5 ml of ethanol. A PFSI membrane is immersedin the solution for at least 24 hours. No color change occurs; the PFSImembrane is still transparent.

The PFSI membrane comprising benzene-1,3-diol, now an optical sensor, istied with parafilm to one end of a gold-coated stainless steel strip.The optical sensor is suspended in a 500 ml round bottom flask and aboveapproximately 0.01 grams of trimellitic anhydride crystals containedwithin the flask. The flask is immediately sealed with a rubber stopper,which pins the steel strip against the mouth of the flask and leaves theoptical sensor suspended above the TMA crystals. The stopper is sealedwith parafilm.

The bottom of the flask is heated through a heating mantle until the TMAcrystals vaporize, yielding a TMA vapor concentration of about 0.25% byvolume. After exposure to TMA, the PFSI membrane is brownish yellow.

EXAMPLE 9

In this example, three optical sensors are prepared according to theprocedure described in Example 8. Using the procedure set forth inExample 1, one optical sensor is exposed to a 10 ppm TMA vapor, oneoptical sensor is exposed to a 10 ppm phthalic anhydride vapor, and oneoptical sensor is exposed to 10 ppm maleic anhydride vapor. The opticalsensors exhibit a unique response to each compound. The optical sensorexposed to TMA exhibits a color shift from transparent to lightyellow-brown; the optical sensor exposed to phthalic anhydride exhibitsa color shift from transparent to tan-light brown; the optical sensorexposed to maleic anhydride exhibits a color shift from transparent tobrown.

EXAMPLE 10

In this example, a solution comprising a transition metal-free dyecomponent is prepared by dissolving 120 mg of 3,4-diaminobenzophenone in12.5 ml of ethanol. A PFSI membrane is immersed in the dye solution forat least 24 hours, until no further color change is observed. Beforeimmersion, the membrane is transparent. When removed, the membrane isred.

A 1000 ppm platinum concentration solution is prepared by dissolving0.03 grams of hexachloroplatinic acid (H₂PtCl₆.6H₂0) in 10 ml of 0.1NHCl. The platinum solution is transferred to a 500 ml round bottomflask.

The dyed, red PFSI membrane, now an optical sensor, is tied withparafilm at one end of a gold-coated stainless steel strip and immersedin the platinum solution. The platinum solution is heated to 70-75° C.through a heating mantle. The heating temperature replicates the typicaloperating temperature of PEM fuel cells. After 15 minutes, the opticalsensor is removed from the platinum solution and rinsed with deionizedwater. After exposure, the optical sensor is black. The color shift,from red, before platinum exposure, to black, after platinum exposure,is irreversible.

EXAMPLE 11

In this example, two optical sensors are prepared according to theprocedure described in Example 10. One optical sensor is immersed in 100ppm platinum solution; the other is immersed in 10 ppm platinumsolutions using the procedure described in Example 10. The magnitude ofthe color shift of the optical sensor is correlated to the platinumconcentration of the platinum solution. The optical sensor immersed inthe 100 ppm platinum concentration exhibits a color shift from red todark brown; the optical sensor immersed in the 10 ppm platinumconcentration exhibits a color shift from red to light brown.

EXAMPLE 12

In this example, two PFSI membranes comprising 3,4-diaminobenzophenoneare prepared as in Example 1. The dyed, red PFSI membrane is an opticalsensor.

An aqueous solution comprising de-ionized water and 3% glucose by volumeis prepared and heated to 80° C. One optical sensor is immersed in thesolution. After exposure, the optical sensor is brown. The color shiftfrom red, before exposure to glucose, to brown, after exposure toglucose, is irreversible. FIG. 2 is a black-and-white photograph of theunexposed optical sensor on the left and the exposed optical sensor onthe right.

While the invention has been described with reference to certainembodiments, it is understood by those skilled in the art that variouschanges may be made and equivalents may be substituted without departingfrom the scope of the invention, as that scope is defined by the claims.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from its scope. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. An optical sensor for monitoring an environmental condition, theoptical sensor comprising a perfluorosulfonate ionomer membranecomprising a solution, wherein the solution comprises a transitionmetal-free dye component, wherein exposure of the optical sensor to aspecific environmental condition produces a color shift on the opticalsensor.
 2. The optical sensor according to claim 1, wherein the solutioncomprises an aromatic-diamine.
 3. The optical sensor according to claim2, wherein the aromatic-diamine comprises 3,4-diaminobenzophenone. 4.The optical sensor according to claim 1, wherein the solution comprisesan azo-based dye.
 5. The optical sensor according to claim 1, whereinthe solution comprises benzene-1,3-diol.
 6. The optical sensor accordingto claim 1, wherein the specific environmental condition comprisespresence of diacetyl.
 7. The optical sensor according to claim 1,wherein the specific environmental condition comprises presence of vinylacetate.
 8. The optical sensor according to claim 1, wherein thespecific environmental condition comprises presence of 2,3-pentanedione.9. The optical sensor according to claim 1, wherein the specificenvironmental condition comprises presence of trimellitic anhydride. 10.The optical sensor according to claim 1, wherein the specificenvironmental condition comprises presence of phthalic anhydride. 11.The optical sensor according to claim 1, wherein the specificenvironmental condition comprises presence of maleic anhydride.
 12. Theoptical sensor according to claim 1, wherein the specific environmentalcondition comprises presence of ionic platinum.
 13. The optical sensoraccording to claim 1, wherein the specific environmental conditioncomprises presence of glucose.
 14. The optical sensor according to claim1, wherein the specific environmental condition comprises pH level inbreath condensate.
 15. A method of monitoring an environmental conditionwith an optical sensor comprising a perfluorosulfonate ionomer membranecomprising a solution, wherein the solution comprises a transitionmetal-free dye component, the method comprising the steps of: exposingthe optical sensor in an environment; and examining the optical sensorfor color shift associated with a specific environmental condition. 16.The method according to claim 15, wherein the examining comprisesvisually inspecting the optical sensor.
 17. The method according toclaim 15, wherein the examining comprises analyzing the optical sensorusing portable or remote spectroscopic analysis.
 18. The methodaccording to claim 15, wherein the monitoring occurs continuously atambient conditions.
 19. The method according to claim 15, furthercomprising emitting an alarm when the optical sensor indicates aspecific environmental condition.
 20. A method of manufacturing anoptical sensor for monitoring an environmental condition, the methodcomprising the steps of: preparing a solution, wherein the solutioncomprises a transition metal-free dye component; immersing aperfluorosulfonate ionomer membrane in the solution; and removing themembrane from the solution after it has absorbed the solution.
 21. Themethod according to claim 20, wherein preparing comprises dissolving anaromatic-diamine in alcohol.
 22. The method according to claim 21,wherein the aromatic-diamine comprises 3,4-diaminobenzophenone.
 23. Themethod according to claim 20, wherein preparing comprises dissolving anazo-based dye in alcohol.
 24. The method according to claim 20, whereinpreparing comprises dissolving benzene-1,3-diol in alcohol.